High security documents such as banknotes and other paper stock have substrates formed from various materials, which may be porous materials such as pulp cotton fibers. Moreover, in the United States, paper currency is made from a non-woven combination of 75% cotton and 25% linen fibers. In most other countries, pulp-based substrates are used. Some countries, such as Canada, have used cotton and paper blended banknotes. In addition, countries such as Australia, New Zealand and Canada have issued banknotes having polymer substrates, e.g., substrates including biaxially oriented polypropylene. The substrate, which may include one or more plies of the substrate material, may include security features such as laminated polymer or paper security threads, planchettes, and watermarks formed directly into the substrate.
As counterfeiters have become more sophisticated, the security features in such documents have had to become more advanced as well in order to prevent widespread fraud. As the substrates of such secure documents have become more advanced, the cost to produce them has also increased, thus making the replacement of worn currency quite expensive. Therefore, it is important that in addition to being secure, such documents must have a high level of durability, lack certain imperfections, and be removed from circulation when the appropriate criteria on their fitness are available. In addition, the measurement and monitoring of porosity and permeability of various media during manufacturing is of importance to obtaining high quality products meeting the required quality.
Banknotes, lottery scratch tickets, and other documents are removed from circulation for a variety of reasons. For example, lottery scratch tickets may be removed if they have pinpricks in the coating. In addition, based on one study, 81% of banknotes are removed because of soiling, 9% are removed because of damage caused by mechanical means, especially tearing, 5% are removed because of graffiti on the notes, 4% are removed because of general wear and tear, and 1% are removed because of damage to the security elements.
Banknotes have a finite time in circulation due to soling and tearing of the notes in use by the public. For example, it takes about 4,000 double folds (first forward and then backward) before a U.S. paper bill will tear. Banknotes are handled in many ways during their usable life and experience a variety of mechanical stresses, as well as being brought into contact with substances that can dirty the notes, resulting in difficulty in their authentication and use.
One important parameter used to determine the fitness of banknotes is limpness. When banknotes have been in circulation, the mechanical wear from folds, handling, and use in bill acceptors, results in a loss of mechanical elasticity that leads to the notes becoming limp. In addition, the mechanical wear of banknotes results in banknotes being torn and/or ripped. This “limpness,” tearing, and ripping has been shown to be directly related to changes in the porosity of the banknote with mechanical wear. In particular, the porosity of the banknotes increases with use and manifests itself in a lower effective elastic constant.
Permeability has been shown to have a correlation to limpness. Studies have also correlated permeability to deflection and stiffness. Permeability is sensitive to network deformation of a substrate, and changes in permeability, typically due to changes in porosity, can be an early indicator of the condition of the substrate network, which itself can be an early predictor of limpness. Existing methods for measuring permeability and porosity, however, are too slow for machine-readable fitness measurements.
Generally, porosity is an important physical parameter for a number of applications and as a diagnostic tool. For example, it plays a critical role in membrane separations, time released drug delivery, soil science and engineering and banknote fitness. In particular, porosity is used in a variety of fields including pharmaceuticals, ceramics, metallurgy, materials, manufacturing, earth sciences, soil mechanics, and engineering.
Typically, porosity and permeability are measured using the transport of liquids or gasses and characterizing the void fraction, physisorption, and tortuosity of the voids in a material or membrane. The detection of the gas or liquid passing through the material or membrane is measured with a variety of methods, including flow meters, mass spectrometers, absorption spectra, fluorescence, mercury intrusion, water evaporation, and mass change, computed tomography.
Specifically, with respect to banknotes, given the large numbers of banknotes in circulation for even small countries, determining the fitness of banknotes is not only of importance in cost control, but also poses a serious technical challenge in terms of processing speed and accuracy. As a result, accurate determination of the fitness of banknotes by measurement of permeability and porosity would be beneficial if it could be performed on the high speed sorters used by commercial and central banks to process currency for authenticity and fitness.
There is, therefore, a need to employ an efficient and accurate manner of identifying whether banknotes, lottery scratch tickets, or other documents are torn, ripped, have been tampered with and/or have been subject to excessive mechanical wear based on the porosity of the documents in order to determine whether the documents should remain in circulation or be destroyed.